The present invention relates to a method of making focusing surfaces, in particular grazing-incidence beam focusing surfaces.
Focusing surfaces of this kind are, for example, WOLTER type I mirror modules that are intended for making an X-ray beam telescope at grazing incidence. The surface of a mirror module is a portion of a surface of revolution (generally a quadric of revolution) about an axis, which is the optical axis of the system. All the numerous modules are concentric (and of a different radius from one module to another) about the same optical axis of revolution. Since the surfaces of revolution of these modules are truncated by two planes perpendicular to the axis of rotation, we will hereafter name these surfaces xe2x80x9ctruncated with aspheric generating lines.xe2x80x9d A method of producing such modules is described, for example, in the article xe2x80x9cThe results of the X-ray mirror modules production for the ESA XXM Spacecraft,xe2x80x9d by D. de CHAMBUIRE, et al., published in the review of the International Conference on Space Optics (ICSO), which was held in Toulouse in 1997. These mirror modules are produced from double conical mandrels, which are machined to the final required form. The number of these mandrels is equal to the required module types.
Since the mandrels of revolution have a shape complementary to the mirrors being made, the final mirror is obtained either by molding, or by fusing a thin sheet of glass, or by replication from resin, or as described in the foregoing article by electrochemical metal plating in the presence of nickel.
The design and construction of these mandrels of revolution generates two disadvantages:
a) For large diameters, their manufacture requires larger and larger machines.
b) It is necessary to construct a new mandrel, when the module diameter of the telescope is varied.
Furthermore, a telescope project, such as the XEUS project of the ESA utilizes a large number of elementary WOLTER type I mirrors, which form among themselves assembled sections to form petals that are arranged according to the modules or rings. The foregoing technique involves the construction of as many mandrels as there are ring types (384 in the XEUS case).
The basic idea of the invention is to make angular conic sections, in particular with a desired aspheric generating line (instead of a linear generating line), for example, an angular section in particular from 10xc2x0 to 20xc2x0, and to design them such that they are obtained by deformation. A conic section is understood to be a portion of a surface, which can be generated by rotating a generating line with a desired shape about an axis of revolution. An angular conic section with an aspheric generating line is understood to be any portion of a surface, which can be generated by rotating an aspheric generating line about an axis of rotation.
Once constructed, the different identical sectors will be assembled, so as to form a module of the telescope. The different modules, each formed from specific sectors, will be arranged so as to retain their common optical axis of rotation. They nest into one another.
The invention makes it possible to avoid the disadvantage a), since the use of sectors makes it possible to limit the dimension of the machines that are used for their construction, as well as disadvantage b), since by applying calibrated loads, it is possible to obtain from a single piece, mirror sectors corresponding to different telescope diameters, or otherwise to differently dimensioned modules or rings of the same telescope.
Thus, the invention relates to a method of making an optical beam focusing surface, characterized in that it uses the generation of, for example, a conic section with a reference contour, and which is provided with a device for deforming the reference contour along at least one direction to obtain a conic section, in particular with an aspheric generating line of a desired shape. Before its deformation, the reference contour may be, for example, plane, or truncated, or even cylindrical.
The method of the invention is suitable for constructing single-surface mirrors as well as mirrors with two angularly offset surfaces and with a shape of a conic section with an aspheric generating line, for example, WOLTER type mirrors.
According to a first variant, the mirror is obtained by deforming the mandrel, so that its contour defines an angular section with a predetermined desired shape that is complementary to that which is desired for the mirror. The mirror is then constructed by molding, replication, or metal plating. A subsequent separation makes it possible to obtain a mirror with the desired shape which is complementary to the desired shape of the reference contour. According to this variant, the mirror is thus obtained directly in its final shape, and the mandrel may be used to construct mirrors with different characteristics, which are obtained by deforming the mandrel in an appropriate manner. One will be able to produce mirrors pertaining to different modules of the same telescope with the same deformable mandrel.
According to a second variant, the mirror is directly obtained by depositing on the mandrel a coating that forms a reflecting surface. The method is characterized by the steps of:
Making a reference contour and a coating that forms a reflecting surface;
Deforming the reference contour according to a predetermined desired shape, so that the reflecting surface defines a mirror according to a sector of the desired shape.
According to this variant, the directly constructed mirror has however a shape which may vary by deformation.
The original surface that forms the reference contour may be plane. However, it may also be truncated (with a straight, spherical, or possibly aspheric generation), or preferably cylindrical, which facilitates greatly the initial polishing.
The deforming device may comprise a means for adjusting the deformation of the reference contour in a transverse direction and/or a longitudinal direction, so as to obtain the desired contour from the original surface.
The method can be characterized in that the reference contour has a first and a second portion that are hinged to each other, the first and the second portion having each a first and a second reference contour, and that:
The first portion is provided with a means for adjusting the deformation of the first reference contour in a transverse direction and/or a means for adjusting the deformation of the first reference contour in a longitudinal direction.
The second portion is provided with a means for adjusting the deformation of the reference contour in a transverse direction and/or a means for adjusting the deformation of the second reference contour in a longitudinal direction.
The zone between the first and the second portion is advantageously provided with a means for adjusting the angle that is formed by the first and second hinged portions.
The invention relates likewise to a mandrel with a reference contour, characterized in that it is provided with a means for deforming this reference contour along at least one direction.
Advantageously, the mandrel comprises a means for adjusting the reference contour along a transverse direction and/or a means for adjusting the reference contour along a longitudinal dimension.
According to a preferred embodiment, the mandrel is characterized in that the reference contour has a first and a second portion hinged to each other, the first and the second portion having each a first and a second reference contour and, advantageously, an adjustable angle between them, so as to produce, for example, sectors of different modules of a WOLTER type telescope, the modules having however a common axis of rotation and common focusing points.
According to an advantageous embodiment, the mandrel has at least one portion with a reference contour, which is adjustable along the transverse direction, and it as two pairs of end legs. In this embodiment, each portion receives at each end a jackscrew for adjusting the deformation in the transverse direction and a jackscrew for adjusting the angle between the two portions, without requiring a specific jackscrew for adjusting the deformation in a longitudinal direction of each portion. In numerous cases, this device may suffice to change from one contour of a ring or module over to that of following rings or modules.